1. Field of Invention
The present invention relates to a light emitting diode (LED) array control circuit, an LED driver circuit, and an LED array control method; particularly, it relates to an LED array control circuit with voltage adjustment function. The present invention also relates to an LED driver circuit and an LED array control method with voltage adjustment function.
2. Description of Related Art
LEDs are widely applied in many applications; as one example, LEDs arranged in an array are used to provide backlight to a liquid crystal display (LCD). Referring to FIG. 1A, for driving an LED array 20, an LED array control circuit 1 is required to provide a proper voltage and current to the LED array 20.
More specifically, as shown in FIG. 1A, the LED array control circuit 1 includes a first power supply circuit 10 which provides a supply voltage VLED to the LED array 20. The LED array 20 includes N LED strings, and each LED string has M LEDs, wherein M and N are positive integers. One end of each of the N LED strings is commonly coupled to the first power supply circuit 10, and the other end of each of the N LED strings is coupled to a corresponding one of N current sources 301. Each current source 301 controls the current through the corresponding LED string, such that as a whole the LED array generates uniform and consistent backlight. A schematic circuit diagram of the current source 301 is shown in FIG. 1B, wherein when the current source 301 operates normally, the current ILED provided by the current source 301 is balanced at ILED=Vref/R.
However, due to variation resulting from manufacture, the voltage across an LED may be different from one another with a variation up to 10%. In other words, a voltage drop across one LED string may be different from that across another LED string with a variation as high as 10%. For example, if each LED string includes 20 LEDs, in a worst case, the voltage variation between two LED strings may be as high as 6 volts. To ensure that all the current sources 301 operate normally, the supply voltage VLED must be high enough to support the LED string with the highest voltage drop, and therefore in the aforementioned example, there may be an excessive voltage up to 6 volts for some LED string(s) with a lower voltage drop. The excessive voltage will fall across the transistor of the corresponding current source, causing unnecessary power consumption and heat dissipation problems.
FIG. 2 shows another prior art, which is different from FIG. 1 in that the transistors and resistors of the current sources 301 are located outside of the chip 31. Nevertheless, the circuit of FIG. 2 operates in the same manner as the circuit of FIG. 1, and both have the same problems of unnecessary power consumption and heat dissipation.
In view of the foregoing, the present invention provides an LED array control circuit with voltage adjustment function to solve the foregoing problems; the present invention also provides an LED driver circuit and an LED array control method with voltage adjustment function.